Drainwater heat recovery will be the example used in the following specification.
(By way of background: measured by weight or surface area, copper sheet is less expensive than copper tube. The term ‘thermal contact conductance’ is used for heat transfer between solids. The rate of heat transfer between solids is a function of: thermal conductivity; contact area; contact pressure; flow rate; and temperature differential.)
Drainwater is a low-level heat source originating from sinks, showers, wash machines and toilets and the like. In addition, because it contains scum, tissues, particles, lumps, hair and the like, it cannot be stored nor can its flow be slowed. As such it requires a heat exchanger that combines low cost and high performance (for fast payback) and non-blocking (for zero maintenance).
Because drainwater must not be slowed or stopped, the exchange of heat therefore is on-the-fly and must be as fast as possible. In a vertical drainpipe, drainwater naturally flows circumferentially down the inner wall of the drainpipe as a ‘falling film’ which is very effective for heat exchange.
A drainpipe heat exchanger must have a relatively large bore to match plumbing code diameter requirements. Drainpipes can be huge (over 6 inch) to accommodate the entire drainwater flow in a large building such as in a hotel, or small (1 to 2 inches) for localized fixtures such as a home sink drainpipe.
Because drainwater flows non-stop, a drainpipe heat exchanger must have a high rate of heat transfer to be economical. Current drainpipe heat exchanger designs use a copper water tube spirally wound on a copper drainpipe which leaves fully 75% of the water pipe's expensive copper out of thermal contact conductance with the drainpipe. This lowers performance and makes for a long payback period.
With energy waste from drained hot water in the USA alone costing about $340 billion annually (according to the US Department of Energy), there is need for a cost effective drainpipe heat exchanger.